This invention relates to a tank for containing liquid materials in an underground buried location which is particularly but not essentially suitable for a septic or holding tank in which sewage products are contained and/or treated. The term "liquid material" used herein is intended to include various liquids such as water, effluent and the like together with liquid containing various solids such as are produced or stored in a sewage waste system.
Septic and holding tanks for sewage materials have been manufactured for many years and generally the available materials from which these tanks are manufactured are concrete, glass fiber reinforced resin materials and more recently rotationally molded plastics materials.
Examples of concrete tanks are shown in U.S. Pat. Nos. 1,677,626 (Frappy); and 1,715,466 (Miller). One example of a tank formed from a glass fiber reinforced material is shown in U.S. Pat. No. 3,426,903 (Olecko). A more recent product related to a molded tank is shown in U.S. Pat. No. 3,741,393 (Estes). A further patent which is not clear about the method of construction is U.S. Pat. No. 4,325,823 (Graham) which mentions the use of "fiberglass" but is more concerned with the technique of operation of the septic system.
For reasons of weight, cost and convenience, molded tanks have become more popular in recent years but suffer from a number of problems which have reduced their effectiveness. Various shapes of molded tanks have been employed. Many tanks are generally rectangular in cross-section with curves of relatively large radiuses joining the four walls defining the top, bottom and two sides. Other shapes include a circular cross-section defining a circular cylindrical tank and a semi-circular cross-section defining a tank with a flat bottom and a semi-cylindrical upper wall.
Firstly a serious problem relates to the difficulty of forming the necessary compartments in the tank, usually achieved by attaching to the interior of a molded tank one or more bulkheads. In this way, the tank is divided into the necessary separate compartments known as septic compartments and an effluent compartment for a septic tank system. In some cases the bulk heads have been inserted into a slide track system. In other cases a welding technique has been attempted. Both techniques have been unsatisfactory particularly bearing in mind that when the tank is buried, the heavy downward weight of the above materials and severe upward forces generated by the buoyancy action of surrounding ground water tends to distort the tank. Any distortion of the tank causes the walls to pull away from the bulkhead with the danger of leaks from one compartment to the next which cannot be tolerated if the septic system is to remain environmentally acceptable.
Associated with this problem is that of developing a design which is capable of providing sufficient strength to the tank so that it can withstand the relatively high forces generated by the downward pressure of the earth and the upward buoyancy pressure of the ground water. In most cases increased strength is provided by an increased thickness of the wall but this is of course economically disadvantageous since the cost of the finished product is to a great extent directly proportional to the amount of material involved in the molding action.